1. Field of the Invention
The present invention relates to a glass composition for an electric lamp, a stem and a bulb for an electric lamp using the glass composition, and an electric lamp using the glass composition.
2. Related Background Art
Conventionally, lead glass has been representative of glass for an electric lamp. However, the lead glass contains about 10% or more of lead oxide (PbO), and thus from the viewpoint of environmental protection, alternative glass compositions having reduced lead content have been proposed. For example, JP 9(1997)-12332 A discloses a glass composition that is suitable for forming by the Danner process. Further, JP10(1998)-324540 A discloses a glass composition that is suitable for forming and capable of suppressing corrosion of a furnace material.
However, conventional alternative glass compositions as described above are too brittle compared with lead glass, and thus the electric lamps are likely to be broken during manufacturing processes. Particularly in recent years, a trend has been toward electric lamps having more complicated shapes. In such a trend, a fluorescent lamp also has been on the market in which a bulb formed into a straight tube by the Danner process or the like is bent into U shape. Further, some cases require processing in which inner portions of a plurality of bulbs are made continuous with each other using another glass tube. Furthermore, manufacturing of a stem also involves post-processing such as sealing of lead wire or the like. These processes, namely processes in which glass that has already been formed further is processed, cause breakage of the glass. Thus, in a mass production process of an electric lamp, it has been requested that such breakage of glass be reduced.
A first glass composition for an electric lamp according to the present invention contains, expressed in mol % (hereinafter, constituents in each composition are given in mol % unless otherwise noted):
(where R represents at least one selected from the group consisting of Li, Na and K, and M represents at least one selected from the group consisting of Mg, Ca, Sr, Ba, Zn and Pb). In the first glass composition, a total amount of CaO and MgO is larger than a total amount of BaO and SrO. The first glass composition has a brittleness index value B determined by the Vickers hardness test of 7,000 mxe2x88x921/2 or lower.
It is known that the brittleness of glass can be evaluated based on the ratio between the respective lengths of an indentation and a crack that are produced by the Vickers hardness test (Japanese Industrial Standards, Z 2244). JP 9(1997)-52729 A discloses an example in which the ratio is used as an index for obtaining glass with high abrasion resistance. The above-mentioned glass composition has a property suited for an electric lamp while achieving a lower level of brittleness.
A second glass composition according to the present invention contains:
in which a total amount of Li2O, Na2O and K2O constitutes 12 to 17%, a total amount of MgO, CaO, SrO, BaO, ZnO and PbO constitutes 2 to 8.5%, and K2O is contained in a larger amount than an amount of Na2O. This glass composition also has lower brittleness and a property suited for an electric lamp.